According to the Thursday report, Gantz was informed by the Shin Bet internal security agency about the hack some five weeks ago, with officials warning him that any sensitive materials on the phone could be used against him.

Gantz dismissed the reporting and speculation that his hacked phone could have contained intimate correspondences or images as "a gossipy, delusional political story".

His party implied that election rival Netanyahu was behind the timing of the Thursday broadcast.

Gantz's Blue and White alliance said the breach took place "a few months ago" and stressed the phone contained "no embarrassing videos".

"Briefings of sources close to Netanyahu and massive spreading of lies prove beyond any doubt who's behind the publication, and why," a statement from the party read.

Yair Lapid, the former finance minister who joined forces with Gantz to form Blue and White, said in a Friday tweet that Netanyahu's "use of sensitive security materials to try and besmirch Benny Gantz proves he's afraid of him".

Netanyahu's office said in a statement that the premier was not updated by the head of the Shin Bet on the affair.

Gantz's alliance, formed in January, is posing a major challenge to Netanyahu's lengthy rule, with opinion polls saying it would win more seats at the April 9 vote than the premier's Likud party.

But the same polls show it would be Netanyahu, not Gantz, who would be able to form a coalition government with right-wing and religious parties.